Projection displays are used for a wide variety of applications, such as producing the pictures viewed on television screens. A typical projection display system includes a number of components, including a display screen, a light source, and an optical path between them. To create the pictures, one or more light sources are provided to emit light when it is needed. The light they produce is then manipulated by a series of optical devices in order to create the visual image. The visual image created along the optical path is then displayed on the display screen, the television screen for example, or another visual display. In most cases, the goal is to produce the best picture possible. The key to producing a desirable visual display, of course, is the configuration of the various optical devices along the optical path. The selection, operation, and configuration of these devices also contribute to unseen characteristics of the system, such as cost, size, and efficient use of system resources.
Several types of projection displays have recently been developed. These new display systems are now becoming much more common, serving as a replacement for the widely-used CRT (cathode ray tube) display, which produces a visual image by producing and directing a stream of electrons at a treated display surface. The stream could only be directed to one point at any given time, but can be systematically swept across the display with such speed as to create the visual impression of a single image. This technology is fairly well-developed, but has reached the point where perceptible increases in quality are difficult to achieve. A CRT also takes up a relatively-large amount of space because the components used for generating the electron stream must be placed at a certain distance from the display screen. Many recently-developed projection display systems, in contrast, feature a much slimmer profile. In addition, projection display systems often produce much cleaner visual images. The combination of these advantages has made such systems immensely popular.
One such projection-display system is commercially available from Texas Instruments of Dallas, Tex. under the trademark DLP® (or Digital Light Processing®). DLP® projection-display systems utilize a digital micromirror device (DMD) in their optical path. The DMD typically includes an array of thousands of tiny mirrors that are used to manipulate colored light originating at an internal light source. Lenses and other components in the optical path adjust the light for use by the DMD, or convey the image it generates to a display plane. The colored light is reflected by the DMD and projected onto a display plane for viewing according to an input image. Projection lenses may be used to magnify and/or focus the image on the display plane.
The general trend in the industry is toward thinner and lighter displays comparable to those available with flat-panel display systems, such as liquid crystal display systems and plasma display systems. In an attempt to develop thinner projection display systems such as those discussed above using a DMD, a combination of lenses and mirrors are used to create a shorter throw distance, which is required for a larger display, within a thinner cabinet form factor. In these systems, the DMD is off axis such that light reflected off the DMD is directed toward a folding mirror that reflects the light from the DMD toward the display plane. By folding the light path, an actual distance traversed by a light beam may be maintained while a physical dimension between the light source and the display plane may be shortened, thereby allowing a longer throw distance, and accordingly a greater magnification, within a given space.
Front projection systems, such as a ceiling mounted projection system, may also utilize a DMD configured off axis relative to the optical axis. In some of these embodiments, particularly a ceiling mounted projection system, the throw distance is typically longer than in rear projection systems.
Placing the DMD off axis, however, increases the projection lenses' instantaneous FOV and, therefore, increases the effect of any aberration of the displayed image, such as astigmatism, lateral color variations, and distortion.
Attempts to correct the image distortion have focused on the use of complicated and expensive optics. Generally, lenses and mirrors having specific and exacting specifications may be used to a certain extent to shape the image, thereby correcting some of the distortions of the image. These optics, typically aspheric molded plastic with complex shapes, are expensive and complicated to fabricate. Furthermore, because the optics must be precisely located along the optical path, many times the cost of production is exceedingly high, thereby becoming uncompetitive with other display technologies.